Investment Securities - Investment Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 29,823	$ 26,701
Unrealized Losses Recorded in AOCI	(839)	(989)
Carrying Value	28,984	25,712
Gross Unrealized Gains	635	683
Gross Unrealized Losses	(182)	(199)
Fair Value	29,437	26,196
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	200	199
Unrealized Losses Recorded in AOCI	0	0
Carrying Value	200	199
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	200	199
Agency \| RMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	25,741	23,022
Unrealized Losses Recorded in AOCI	(761)	(897)
Carrying Value	24,980	22,125
Gross Unrealized Gains	565	606
Gross Unrealized Losses	(150)	(158)
Fair Value	25,395	22,573
Agency \| CMBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,882	3,480
Unrealized Losses Recorded in AOCI	(78)	(92)
Carrying Value	3,804	3,388
Gross Unrealized Gains	70	77
Gross Unrealized Losses	(32)	(41)
Fair Value	$ 3,842	$ 3,424

Unrealized losses recorded in AOCI on held to maturity securities.

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- Definition

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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